The present invention is concerned with brake apparatus for railway vehicles, and particularly to such brake apparatus as disclosed in U.S. Pat. No. Re. 30,408, assigned to the assignee of the present application.
In the above-mentioned Reissue patent, there is disclosed a dual-piston brake cylinder device that is arranged to provide storage of sufficient air as necessary to develop the desired brake forces, without the need for conventional separate air reservoirs. The dual pistons are tandem-connected, one being larger than the other. The larger piston cooperates with the brake cylinder body to form air storage chambers on its opposite sides.
During brake release, air is supplied to these storage chambers by a control valve device that is operative in response to variations in train brake pipe pressure. When a brake application is initiated, the control valve device connects air from one side of the larger piston to a corresponding side of the smaller piston. A one-way check valve device is provided to effect pressure equalization between the opposite chambers of the larger piston to permit movement of the dual pistons in a brake-application direction under the influence of the pressure acting on the smaller piston. Once the tandem-connected pistons are in application position, the control valve releases pressure from the opposite side of the larger piston to establish a pressure differential thereacross and, accordingly, produce a braking force.
In order to develop consistent braking forces from a wide range of initial pressures corresponding to the desired train charge, it was believed necessary in the development of the foregoing brake apparatus to design the larger piston, hereinafter referred to as the power piston, with equal effective pressure areas subject to the air stored under pressure in the chambers on the opposite sides of the power piston. Such a design is shown in U.S. Pat. No. 4,339,155, issued July 13, 1982, and assigned to the assignee of the present application. If either side of the power piston had a different pressure area than the other, the piston output force would vary with identical pressure reductions, when making these pressure reductions from different initial pressures. This, in turn, would result in the power piston force being pressure sensitive because it would vary by the product of the area difference and the prevailing pressure.
In order to assure equal effective pressure areas across the power piston, as provided in U.S. Pat. No. 4,339,155, it will be noted that the diameter of the front dynamic seal, which seals chamber 14 from atmosphere, is exactly the same as the rear dynamic seal, which seals chamber 13 from chamber 11. In making these seals the same diameter, there is no opportunity to "nest" or "telescope" them for the purpose of saving space. Each of the aforementioned dynamic seals, in addition to the power piston seal itself, requires sufficient longitudinal distance to accommodate the maximum piston stroke. Consequently, the minimum longitudinal dimension of the brake cylinder device employed in the foregoing brake apparatus is determined by the number of dynamic seals (three) multiplied by the maximum piston stroke.
Therefore, in order to limit the longitudinal dimension of the brake cylinder so as to fit within the space normally available, the maximum piston stroke was selected to be approximately 6", or in other words, one-half the stroke of a standard AB/D-type brake cylinder having a 12" piston stroke.
Since the brake rigging on freight brake cars is designed to accommodate the standard 12" piston stroke of an AB-type brake cylinder, the brake apparatus as shown in U.S. Pat. No. 4,339,155, having the short 6" piston travel, requires the present brake rigging and/or lever ratio to be changed to accommodate the shorter piston travel. This entails an additional expense which offsets and adversely affects the projected economic advantages of the combined brake cylinder/air reservoir device when used to retrofit existing AB/D-type brake equipment.